This invention concerns a vacuum filter system for removing solids such as grit and particles from contaminated liquids such as machine tool coolants and other process liquids. The invention also provides corresponding methods.
Replacing and disposing of contaminated process liquids such as machine tool coolants can be costly. Hence, those who use such liquids often seek to reuse them after they become contaminated. Many such contaminated liquids require filtration prior to reuse.
The prior practice of using settling ponds to recondition contaminated liquids presents an undesirable environmental problem, among others.
Prior filtering equipment for conditioning contaminated liquids includes batch filter systems that typically employ individual filter units, e.g. cartridges or bag filter elements, that require periodic maintenance and/or replacement. Such batch systems are ill suited to continuous-production environments.
In contrast, continuous filter systems employ bulk filter media, typically in roll form. Such bulk filter media has a large amount of contiguous filter material, only a portion of which is disposed to filter contaminated liquid at a given time. Bulk filter media systems are available in at least three basic types, namely pressure systems, vacuum systems, and gravity systems.
Pressure filter systems have various sealing assemblies to seal the filter media against a sealing structure. Such sealing assemblies contain the liquid within the filter itself and inhibit contaminants from entering the filtered liquid section. These pressure filter systems can provide high differential pressure, typically in the order of 2,100 grams per square centimeter (g/cm.sup.2) or more, across the media, and consequently can provide good filtration. However, the sealing arrangement adds cost and complexity.
Vacuum filter systems can provide moderate differential pressure, e.g., 350 g/cm.sup.2, across the filter media for good filtration for a given filter area and flow rate. For example, filter areas of 0.5 to 3 square meters and flow rates of 100 to 450 liters per minute per square meter of filter area are common. Vacuum filters can provide reliable automatic operation in demanding environments. As is the case with pressure filters, vacuum filter systems commonly interrupt the liquid flow across the media to advance the filter media.
In gravity filter systems, an input pool of contaminated liquid rests on a filter media. A belt or chain, often mesh-like, supports and advances the filter media. The liquid flows through the media in a manner similar to coffee flowing through a coffee filter, and filtered liquid collects in a filtered liquid tank below the filter media. As the filter media becomes loaded with contaminants, the liquid level rises in the contaminated liquid tank, signaling the need to advance the media and the belt. Typically the depth of liquid in the input pool is in the order of 10 centimeters. A small depth of liquid in the input pool yields a low differential pressure across the media, e.g., usually less than 35 g/cm.sup.2. While gravity filters are relatively simple and inexpensive, they require a large filtration area or a less restrictive filter media, compared to pressure and vacuum filter systems, in order to achieve the same flow rate. However, a less restrictive filter media compromises filtration effectiveness. In addition, gravity filters provide modest contaminant loading per square foot and a high rate of media consumption relative to pressure systems and to vacuum filter systems. A high rate of media consumption in a filter system adds to the expense and maintenance requirements of the filter system. One example of a gravity filter is disclosed in U.S. Pat. No. 5,221,468 issued to Fox et al., and incorporated herein by reference.
A feature common to many of the above described filter systems is a liquid entry port for receiving contaminated liquid from a machine tool. Most machine tools have their work surface at a height that accommodates the operator. The machine tool then discharges contaminated liquid below the work surface, at anywhere from 15 to 100 centimeters above the floor. Thus, if the liquid entry port height of a filter system is above the discharge from the machine tool, e.g., 100 centimeters or higher, a pump is needed to transfer the contaminated liquid to the filter. Such a pump adds to the cost of the filter system.
Electrical controls and electromechanical devices are another feature common to many prior filter systems. These controls and devices typically include an electric motor, a starter, a gear reducer drive, overload protection, and an electric float switch, all of which must meet rigid industrial electrical specifications.
Thus, a need exists for an inexpensive, simple, and effective filter system that has minimal environmental impact.
More particularly, a need exists for a filter system that meets the following criteria: low capital cost; effective filtration, i.e., filtration that removes a large majority of the contaminants from the contaminated liquid; minimal maintenance and operator intervention; modest space requirements; adequate liquid capacity; adequate liquid flow and pressure; long filter media life and modest media cost; and a simple interface with devices that use process liquids, e.g., machine tools.
Thus, it is an object of the invention to provide an effective filter system that is inexpensive and easy to maintain.
It is another object of the invention to provide a filter system that has modest space requirements and operates without the need for a pump to transfer liquid from a machine tool to the filter system.
It is another object of the invention to provide a filter system of the character described above that does not require extensive electrical controls.
Other objects of the invention will in part be obvious and will in part appear hereinafter.